Gel shaped articles made of a polymer material have been intensely studied as a biocatalyst carrier, a water retention agent, a refrigerant, an alternative to a biological gel for an eye, a skin and a joint, a sustained-releasing material for a drug and a substrate in an actuator. Examples of a polymer material for these gel shaped articles include agar, alginates, carrageenan, polyacrylamides, polyvinyl alcohols (PVAs) and photocurable resins. A carrier used for treating waste water must have a higher water content, permeability for oxygen and a substrate and higher biocompatibility. A PVA is a particularly excellent material meeting these conditions.
Patent Reference Nos. 1 to 5 have described a process for manufacturing a gel shaped article as a carrier for treating waste water and a carrier for a bioreactor.
Patent Reference No. 1 has described a process wherein an aqueous mixed solution of a PVA and sodium alginate is contacted with an aqueous solution of calcium chloride to form a spherical PVA shaped article which is then frozen/thawed. Patent Reference No. 2 has described a process wherein an aqueous solution of a PVA is injected into a template, frozen and then partially dehydrated.
The processes described in Patent Reference Nos. 1 and 2 are those in which a PVA is insolubilized by forming microcrystals of a PVA by means of freezing/thawing or freeze-drying. Such physiochemical crosslinkage is, however, so weak that it can be broken by a PVA-decomposing bacteria such as Pseudomonas.
Patent Reference No. 3 has described that a PVA shaped article is produced from a PVA-containing solution, and then is contacted with an aqueous solution containing an aldehyde for crosslinking, to form a gel shaped article. Examples 1 to 4 therein specifically describe a process where a PVA shaped article is contacted with formaldehyde. However, this process requires that an aqueous solution to be contacted with a PVA shaped article contains an aldehyde, leading to a large amount of the aldehyde. Furthermore, a PVA eluted from the PVA shaped article is precipitated in a reaction bath, causing clogging of a vessel, pipes, a pump and so forth during the production process. Furthermore, shrinkage of a PVA shaped article in the course of the production process leads to reduction in a water content, that is, reduction in a microbial habitat, so that the article becomes less preferable as a carrier for treating waste water or for a bioreactor. Furthermore, Example 5 in Patent Reference No. 3 describes a process where a PVA shaped article is contacted with an aqueous solution containing glutaraldehyde, but a PVA shaped article produced by this process is not porous. Therefore, when the PVA shaped article is used as a microorganism carrier, a microbial habitat is unfavorably limited to the surface of the PVA shaped article.
Patent Reference No. 4 has described a process for producing a spherical gel shaped article wherein an aqueous mixed solution of a PVA, sodium alginate and glutaraldehyde is added dropwise to an acidic aqueous solution containing polyvalent metal ions with a pH of 3 to 5. However, when the acidic aqueous solution has a pH of 3 to 5, a reaction of a PVA with the aldehyde takes a long time, leading to the problem that a PVA and the aldehyde are eluted into an acidic aqueous solution. A PVA shaped article produced by the process described in Patent Reference No. 4 is also non-porous. Therefore, when the PVA shaped article is used as a microorganism carrier, a microbial habitat is unfavorably limited to the surface of the PVA shaped article.
Patent Reference No. 5 has described a PVA shaped article having a particle size of 1 to 20 mm and an acetalization degree of 1 to 20 mol %, which is produced by reacting polyvinyl alcohol with a dialdehyde-containing solution to form a shaped article and contacting the article with an acid having a pH of 3 or less. According to this production process, the use of glutaraldehyde leads to reduction in the amount of an aldehyde, so that the problems of brittleness and shrinkage during the production process can be improved. However, since a PVA shaped article obtained is non-porous, a microbial habitat is unfavorably limited to the surface of the PVA shaped article.